Tempered threaded members and method of making

ABSTRACT

1,067,184. Making threaded members. CATERPILLAR TRACTOR CO. Nov. 4, 1965 [Nov. 27, 1964], No. 46736/65. Heading B3A. [Also in Division F2] A threaded member, e.g. a bolt or stud, is formed with a hardened core and a tempered outer casing in which the threads are formed. The threads are formed either in the throughhardened blank before tempering or the blank is tempered before the threads are formed.

Jan. 31, 1967 c. D. LOYD 1 3,301,120

TEMPERED THREADED MEMBERS AND METHOD OF MAKING Filed Nov. 237, 1964 2 Sheets-Sheet 1 Fig.1.

INVENTOR.

CALAHN D. LOYD' WMJW AT TORNEYS c. D. LOYD 3,301,120

TEMPERED THREADED MEMBERS AND METHOD OF MAKING Jan. 31, 1967 2 Sheets-Sheet 2 Filed Nov. 27, 1964 RADIALD (INCHES $0. x .m& Ikdzmmkm udmzmk INVENTOR. CALV N D. LOYD AT'I' ORNEYS United States Patent 3,301,120 TEMPERED THREADED MEMBERS AND METHOD OF MAKING Calvin D. Loyd, Peoria, 111., assignor to Caterpillar Tractor (10., Peoria, 111., a corporation of California Filed Nov. 27, 1964, Ser. No. 414,208 7 Claims. (Cl. 851) This invention relates to threaded metal members and more particularly to threaded members that are case tempered.

The ever greater mechanical stresses that machinery is being called on to withstand have created a demand for bolts, studs, and other threaded fasteners of increased strength over that heretofore demanded. Threaded members are especially prone to stress failure because of the very nature of their configuration. Specifically, the thread configuration with its attendant notches and acute angles creates lines of concentrated stress from which cracks are initiated and high internal stresses are set up.

Much work has been done in an attempt to improve the strength of threaded members through the use of special alloys resistant to crack propagation and through careful attention to the heat treatment of thread stock to produce uniform physical properties throughout crosssectional area of the member. Also much attention has been paid to the careful design of thread contours in an attempt to minimize stress concentration. However, the very nature of threads tends to defeat these attempts and there is still a great demand for threaded members of increased strength but utilizing well-known and readily available alloys and simple heat treatment methods.

It is an object of the present invention to provide threaded members of .improved strength having a hardened core portion and a tempered outer casing of a depth greater than that of the threads thereon.

It is another object of the invention to provide a process for producing improved threaded members having a hardened core portion and a tempered casing.

It is a further object of the invention to provide threaded members having increased resistance to crack propagation and the concentration of stresses within the member structure.

The invention has other objects and advantages that will become apparent from an understanding of the following specification wherein reference is made to the accompanying drawings.

In the drawings:

FIG. 1 is a schematic longitudinal sectional view through a stud made in accordance with the present invention;

FIG. 2 is a schematic cross section through a threaded member made in accordance with the invention and further illustrating the hardness at various radii from the center line of such threaded member;

FIG. 3 is a schematic illustration of a through-hardened notched test specimen and an accompanying graph illustrating the hardness and tensile strength gradients in a cross section through said test specimen;

FIG. 4 is a schematic illustration of a notched test specimen made according to the present invention and an accompanying graph illustrating the cross-sectional hardness and tensile strength thereof; and

FIG. 5 is a schematic longitudinal sectional view through a bolt made in accordance with the invention.

Most conventional threaded members common to the art are through-hardened to a substantially constant hardness gradient. In such through-hardened threaded studs or bolts, which are normally loaded in tension and/or bending, the notches created by the threads set up stress concentrations whereby the ultimate strength of the threaded member is substantially reduced. In such 3,301,126 Patented Jan. 31, 1967 through-hardened threaded members a major portion of the total stress in the member is concentrated at the roots of the threads. It is further well known that a notch induces brittle behavior in steels, and the consequence of the above structure and through-hardening is the origination of cracks in the thread roots when the member is loaded. These cracks eventually propagate through the body of the threaded member and failure of the stud or bolt results.

In the present invention threaded members are provided which have increased ultimate strength as well as high resistance to crack origination and propagation. Such threaded members of the present invention are provided with a hardness gradient extending from the peripheral surface of the stud or bolt through the area of the threads and into the central core portion of the member. More specifically the case portion of the threaded member is tempered, i.e., reduced in hardness, while the central core portion of the member remains hard. The presence of a hardness gradient in the threaded portion of the member reduces stress concentration in the thread roots and thereby in turn reduces the brittle behavior of the metal normally caused by notches.

More specifically, such a case tempered threaded member is illustrated in FIG. 1 of the drawings wherein a stud 11 has a hardened central portion 12 comprising the main body of the member. The stud 11 is further provided with an outer case portion 13 that has been tempered to a hardness somewhat less than that of core portion 12.

Conventional threads 14 are formed into the peripheral surface of stud 11. The tempered case 13 extends for a distance radially inward from the surface of stud 11 and eventually meets and merges into the untempered hardened core portion 12. While FIG. 1 illustrates a sharp discontinuity in hardness between outer tempered case 13 and hardened core portion 12, in actuality the tempered case merges into hardened core portion 12 as illustrated in FIG. 2 of the drawings. FIG. 2 is an illustration of the cross-sectional hardness of a typical threaded member of the invention. The large dots thereon with the indicating numerals illustrate actual points at which the hardness of such cross section has been tested, and the indicating numerals indicate the Rockwell C hardness at those particular points in the cross section. Thus it can be seen that in this typical cross section, core portion 12 has a Rockwell C hardness varying from about 44 at center portion thereof to about 46 at the case interface. In addition it can be seen that case 13 has a Rockwell C hardness varying from about 35 at the outermost portion thereof to about 41 as the case approaches the interface with the hardened core 12.

It is further important to note that tempered case 13 extends inwardly further into stud 11 than the depth of roots 16 of threads 14.

The relatively soft outer case 13 provides a more ductile material in the root area of the threads to reduce notch sensitivity and stress concentration in stud member 11. As stud member 11 is loaded in tension and/or bending, the material in the thread roots 16 yields slightly to eliminate stress concentration. This yielding permits .the stronger, relatively harder, unnotched core portion 12 to support the load.

The threaded members of the present invention may be fabricated from any conventional stud or bolt steel, it being necessary only that the steel be susceptible to conven* tional hardening and tempering processes.

Hardened core 12 and tempered case 13 of the threaded member are produced by well known conventional heat treatment methods, it being necessary only that the previously hardened stock be case tempered to a depth greater than the ultimate depth of the threads to be provided thereon.

As an illustration of the method for producing the Case tempered threaded members of the invention, one inch bars of S.A.E. 8630 steel were hardened and case tempered in the following manner: The bars were 4 were then threaded by conventional techniques to produce threaded test studs. The strength of these case tempered studs was then compared to studs of the same alloy wherein one stud was a production stud, another stud had heated to a temperature of approximately 1560 F. and 5 been nitrided for hours, and a third case hardened thereafter water quenched to through-harden the bars. stud. All of the studs were then tested for strength both The hardened bars were then tempered at 400 F. to a in tension and in a wedge test, which simulates conditions hardness of Rockwell C 46. The through-hardened bars of misalignment and is a measure of ductility, as per were then case tempered by induction heating the outer S.A.E. specifications. The test results were as follows:

TABLE I no -RkllC Ul'tSt 1 tud Tested ar ness 0c we tnnfrpesmrengti Core Case Tension Wedge Tension Case tempered studs per invention:

1 45-46 3343s 208, 000 208, 000 2 45-46 33-33 209, 000 211, 000 3.... 45 1s 33*38 207, 000 Production stud 34-38 34-38 162, 000 160, 000 N itrided stud. 32-34 55 143, 000 147, 000 Cas harden d stu 34-38 50 168, can 153, 000

portion thereof to approximately 1300 F. followed by a water quench. This resulted in a case hardness of approximately Rockwell C 33 to 38 to a depth of approximately 0.125 inch. Threads were then conventionally machined into the case tempered portion of the bars.

Notched test specimens as illustrated by numeral 21 in FIG. 4 of the drawings, were produced by the process as illustrated supra. These case tempered notched test specimens were compared with a conventional throughhardened production stud test specimen as illustrated at numeral 22 in FIG. 3 of the drawings. Immediately below the test specimens 21 and 22 in FIGS. 4 and 3 respectively, of the drawings is reproduced a graph of a stress diagram and the cross-sectional hardness gradient of the specimen immediately thereabove. The solid line indicates the stress diagram for its respective test specimen, while the broken line indicates the specimen hardness. It should be noted that the hardness gradient line 23 for conventional production specimen 22 indicates that the hardness remains constant over the entire cross section of the specimen. This illustrates the conventional through-hardness condition of the stud. Hardness gradient line 24 of specimen 21 produced by the method of the present invention indicates that there is a tempered case portion wherein the hardness decreases from that shown by the core portion to a somewhat softer condition in the peripheral areas thereof. The horizontal portion 26 of the stress diagram for case tempered test section 21 and the horizontal portion 27 of the stress diagram for conventional test specimen 22 indicate the ultimate tensile strength of the respective specimen. Thus it can be seen that the test specimen produced according to the present invention and having a case tempered portion had an ultimate tensile strength of approximately 260,000 pounds per square inch, where- ;as the test specimen produced by conventional throughhardening methods had an ultimate tensile strength of only 160,000 pounds per square inch.

As a further illustration indicating the greater strengths obtained in threaded members having a case temper according to the present invention over studs produced by conventional production methods or other special means, reference is made to Table I below. In this instance zthree one-inch bars of S.A.E. 8360 steel were heated to 1560 F. and then water quenched. The quenched bars were subsequently tempered at 400 F. to an overall hardness of Rockwell C 46. The bars were then case tempered by induction heating in a 9,600 cycle Tocco induction machine with a power setting of 30kilowatts and ascanning speed of approximately one inch per second with attendant w ter @Ooling. The case tempered bars As illustrated in Table I, the case tempered studs produced according to the present invention have significantly higher ultimate strength both in tension and wedge tension than studs produced by other methods. In addition it was noted that despite the high strength of the studs of the invention, appreciable elongation occurred in the threads.

It should be understood that while specific test illustrations have been given of studs produced according to the present invention, other types of threaded members are equally benefited when made with a case tempering according to the present invention. For instance, case tempered bolts demonstrate improved physical properties when provided with a tempered case both at the threaded portion thereof as well as at the head portion. With reference to FIG. 5 of the drawings there is illustrated a bolt 28 having a head portion 29 and a threaded shank portion 30. The outer periphery of shank portion 30 has been tempered to form a softer casing 31 in which threads 32 are rolled. Casing 31 extends throughout the length of shank portion 30 and also extends around shoulder 33 where head 29 meets shank 30.

Case tempering of the head end of the bolt where it meets the shank portion greatly reduces the stress concentration in that area whereby a bolt of improved properties is provided. As an illustration some bolts produced according to the present invention and having a case hardened portion at the head end as well as the threaded section, when tested according to S.A.E. specifications, exhibited yield strengths of as high as 188,300 pounds per square inch and ultimate strengths of as high as 224,600 pounds per square inch. Such bolts also exhibited a breaking stress of as high as 224,100 pounds per square inch in a wedge tension test.

It will be appreciated that the conditions presented supra illustrating the heat treatment for producing a case tempering on bar stock may be varied considerably depending upon many factors. Thus the use of different steel alloys may affect the tempering temperatures, as will also the diameter of threaded members to be produced, and the depth of threads to be provided. It is only necessary that the tempered case portion of the threaded part extend to a depth greater than the root depth of the thread, as previously described. The provision of such a tempered case on a particular alloy to a particular depth is well within the skill of those engaged in the art.

It will also be recognized that an alternative method of producing the case tempered threaded members of the invention may be employed. Specifically, threads may be machined or rolled into untreated bar stock,

Subsequent to the thread forming operation, the threaded stock is heated and quenched to through-harden the piece.

The threaded through-hardened stock is then induction tempered and quenched to provide a tempered case portion. It is only necessary to temper the threaded member to a depth greater than the root depth to the threads. However, this can be accomplished readily by those skilled in the art.

It should be recognized that when threaded members are produced by this lastly described method, the casing depth will vary somewhat axially along the member due to the shielding effect of the thread crests. However, so long as the temper extends deeper than the thread roots into the core portion, the beneficial effects of the present construction will be realized.

What is claimed is:

1. A threaded steel member having a circular crosssection and high resistance to cracks and crack propogation, comprising:

(a) a through-hardened core portion of substantially circular cross-section, said core portion extending radially of the member from the axis thereof to adjacent the periphery thereof and having a Rockwell hardness substantially higher than that of the unhardened steel of which said member is composed;

(b) a tempered casing constituting the outer peripheral portion of said member, said casing extending radially inward from the periphery of said member and merging into said core portion, the hardness of said tempered casing decreasing from that exhibited by said core portion to a substantially softer condition through said peripheral portion; and

(c) threads formed in said tempered casing portion, the root depth of said threads being substantially less than the cross-sectional depth of said tempered casing portion,

said threaded member being characterized by a significantly higher ultimate strength in tension and wedge tension than that exhibited by a threaded member composed of the same steel but having a case hardness which is the same or greater than its core hardness.

2. A high strength threaded steel member having a circular cross-section, comprising:

(a) a hardened core portion of substantially circular cross-section, said hardened core portion extending radially of the member from its axis to adjacent its periphery and having a Rockwell hardness substantially higher than that of said member prior to hardening thereof;

(b) a casing constituting the outer peripheral portion of said member, said casing constituting a tempered portion softer and more ductile than said core portion and merging into the latter; and

(c) threads formed in said casing portion, the root depth of said threads being substantially less than the cross-sectional depth of said casing portion,

said threaded member being characterized by a significantly higher ultimate strength in tension and wedge tension than that exhibited by a threaded member composed of the same steel but having a case hardness which is the same or greater that its core hardness.

3. A threaded member as defined in claim 2, wherein said member is in the form of a stud.

4. A threaded member as defined in claim 2, wherein said member is in the form of a bolt and wherein said casing extends throughout the length of the shank portion of the bolt and around the shoulder formed at the juncture of the shank portion and the head portion of the bolt.

5. A process for producing high strength solid, threaded members, comprising the steps of:

(a) through-hardening a solid blank member of steel of circular cross-section to a Rockwell hardness sub stantially higher than that of said blank;

(b) tempering the outer peripheral portion of said hardened blank to impart a softer, more ductile state thereto; and

(c) forming threads into said softer, more ductile outer portion, said threads being formed with a root depth substantially less than the cross-sectional depth of said outer portion.

6. A process for producing high strength solid, threaded members, comprising the steps of:

(a) heating and subsequently quenching a solid blank of steel of circular cross-section, to through-harden the blank to a Rockwell hardesss substantially higher than that of said blank;

(b) re-heating the outermost peripheral portion of said through-hardneed blank to form a softer, more ductile case thereon; and

(c) forming threads into said softer, more ductile case, said threads being formed to have a root depth substantially less than the cross-sectional depth of said case.

7. A process for producing high strength solid, threaded members, comprising the steps of:

(a) forming threads into the peripheral surface of a solid blank member of steel of circular cross-section;

(b) heating and quenching said threaded member to through-harden the member to a Rockwell hardness substantially higher than that of said blank member; and

(c) tempering the thus hardened member along the threaded portion thereof to a depth substantially greater than the root depth of said threads.

References Cited by the Examiner UNITED STATES PATENTS 86,220 1/ 1869 Frost. 782,467 2/ 1905 Replogle. 2,340,349 2/ 1944 Somes. 2,340,706 2/1944 Somes. 2,511,706 6/1950 Gibbons 14812.4 2,730,472 1/1956 Osborn 148--12.4

CARL W. TOMLIN, Primary Examiner. MARION PARSONS, IR., Examiner, 

1. A THREADED STEEL MEMBER HAVING A CIRCULAR CROSSSECTION AND HIGH RESISTANCE TO CRACKS AND CRACK PROPOGATION, COMPRISING: (A) A THROUGH-HARDENED CORE PORTION OF SUBSTANTIALLY CIRCULAR CROSS-SECTION, SAID CORE PORTION EXTENDING RADIALLY OF THE MEMBER FROM THE AXIS THEREOF TO ADJACENT THE PERIPHERY THEREOF AND HAVING A ROCKWELL HARDNESS SUBSTANTIALLY HIGHER THAN THAT OF THE UNHARDENED STEEL OF WHICH SAID MEMBER IS COMPOSED; (B) A TEMPERED CASING CONSTITUTING THE OUTER PERIPHERAL PORTION OF SAID MEMBER, SAID CASING EXTENDING RADIALLY INWARD FROM THE PERIPHERY OF SAID MEMBER AND MERGING INTO SAID CORE PORTION, THE HARDNESS OF SAID TEMPERED CASING DECREASING FROM THAT EXHIBITED BY SAID CORE PORTION TO A SUBSTANTIALLY SOFTER CONDITION THROUGH SAID PERIPHERAL PORTION; AND (C) THREADS FORMED IN SAID TEMPERED CASING PORTION, THE ROOT DEPTH OF SAID THREADS BEING SUBSTANTIALLY LESS THAN THE CROSS-SECTIONAL DEPTH OF SAID TEMPERED CASING PORTION, SAID THREADED MEMBER BEING CHARACTERIZED BY A SIGNIFICANTLY HIGHER ULTIMATE STRENGTH IN TENSION AND WEDGE TENSION THAN THAT EXHIBITED BY A THREADED MEMBER COMPOSED OF THE SAME STEEL BUT HAVING A CASE HARDNESS WHICH IS THE SAME OR GREATER THAN ITS CORE HARDNESS. 